I think that clip is so that the needle valve will not stick in the seat. Mechanical fuel pump pressure is around 4 p.s.i. which isn't much, but could be a problem after the vehicle has been sitting awhile. I would'nt put a priority on it if you don't have one, I have not heard of it being a problem. I'm sure it could stick though. Maybe you could find a carb shop that would have one or even make it.